Traditional quality control methods for viscosity measurement of adhesive and sealant materials focus on the single point test. Using a rotational viscometer with disc-type spindle or T-bar spindle (or perhaps even a cone/plate model), the technician runs the instrument at one speed, records the viscosity number, and verifies that it falls between maximum and minimum limits for acceptance. Failure requires a repeat of the test on a fresh sample or, if there is concern that the instrument might be the problem, a calibration check on the viscometer is performed with a viscosity standard fluid.
Increased capability designed into today’s generation of benchtop viscometers and rheometers has created opportunity for more comprehensive viscosity measurements in roughly the same amount of test time. Assessing how viscosity changes when the material is tested at different rotational speeds gives a complete picture of flow behavior. Low rotational speeds of the viscometer correlate with how the adhesive/sealant settles into position after application, while high rotational speeds simulate spraying, rolling, squeezing or scraping the material onto the substrate surface. Testing flow behavior over a range of rotational speeds is the smart approach in today’s QC lab because it mimics the way that customers will try to use the adhesive or sealant in actual practice.